As a planar illumination device, a direct type illumination device and an edge-light type illumination device are known. In the direct type illumination device, as can be seen in a backlight for a liquid crystal panel for example, a diffusive plate formed of a plate having a certain thickness and area is used, a light source such as a fluorescent light is disposed right below the diffusive plate, and a surface of the diffusive plate emits light as the diffusive plate is directly irradiated with light from the light source. In the edge-light type illumination device, a light guiding plate formed of a plate having a certain thickness and area is used and a light source such as a fluorescent light or an LED is disposed on at least one side of the light guiding plate to cause a surface of the light guiding plate to emit light.
Of the direct type and the edge-light type illumination devices, the direct type illumination device has a structure in which a specific gap, i.e., a specific distance is provided between the light source and the diffusive plate. Thus, if the distance is short, the outer shape of the light source and the like may be projected on the diffusive plate to give unpleasant appearance and degrade the illumination quality. If a highly directional light source is used, uniform illumination light may not be obtained because the brightness of a portion of the diffusive plate right above the light source becomes extremely high to generate a difference in brightness between the portion and other illumination areas. As a method of uniformizing the brightness, a large distance may be provided between the diffusive plate and the light source. However, the use of this method may cause problems that desired illumination light cannot be obtained because the entire area proportionally becomes darker as the distance becomes larger, or the device cannot be made thin, for example. Such problems in the direct type illumination device make it difficult to employ the device depending on the use.
Because the direct type illumination device has the problems described above, the edge-light type illumination device is used instead of the direct type illumination devices and numerous illumination devices of this type have been proposed (see, for example, Patent Documents 1 to 3).
For example, an edge-light type illumination device is disclosed in Patent Document 1. The illumination device is configured by including an LED, a light guiding plate having a size of about a postcard in which a light guiding part is formed to have a flat surface, and a reflecting mirror that reflects light from the LED, installing the LED on the flat surface of the light guiding plate, and covering the LED with the reflecting mirror. Thus, the light emitted from the LED is reflected by the reflecting mirror to be guided to the light guiding plate. With the illumination device, the light emitted from the LED is efficiently taken into the light guiding plate.
An illumination device including a light source device formed of an LED and a light source rod, and a light guiding plate guiding the light emitted from the light source device is disclosed in Patent Document 2. The light source rod is formed of a prism array of a prescribed shape. The light source rod causes an irradiation target to be irradiated with the light emitted from the LED via the light guiding plate to uniformize the brightness.
Patent Document 3 discloses a register guide lamp in which a plurality of LEDs are disposed on a light entering surface of a light guiding body at an equal interval. Light from the LEDs is irregularly reflected by a reflector, and the irregularly reflected light causes plane emission of light by the light-emitting surface of the light guiding body, whereby a display body disposed facing the light-emitting surface of the light guiding body is illuminated.